Can I identify oil thickness or thinness by checking only Viscosity (cSt) at 40°C/104°F?

[vikz]

Hi,

I'm new to this forum. I have a question on the lubricant viscosity index and Viscosity (cSt) at 40°C.

Can we identify the oil thickness or thinness by comparing only the Viscosity (cSt) at 40°C? Without checking the viscosity index (VI)?

Some oils have a lower Viscosity (cSt) at 40°C (around 34 -36), but such oils have a higher viscosity index like 174. For example, Castrol Transmax CVT. Though some oils have a higher Viscosity (cSt) at 40°C (like 128), they have a similar or lower viscosity index like 102. (most of the hydraulic and chainsaw bar oils) Bellow, I linked these two products' datasheets.

So how do I verify one oil is thinner or thicker in this type of situation? Why do transmission oils have a higher viscosity index and lower Viscosity (cSt) at 40°C?

These are the lubricant manufacturers' data sources that I used.

• Castrol Transmax CVT data sheet. (link).
• Chainsaw Bar Oil data sheet (link).

I'm asking this question not to interchange these two lubricants. What I want is to improve my knowledge.

 

[ron17571]

I would add looking at the owner's manual and carefully matching the requirements of the manufacturer. My Ford's c-6 trans and my minivan trans had very specific specs and special fluid needs.

 

[Astro14]

No.

The relationship between viscosity and temperature is a curve. The shape of which varies depending on formulation.

You cannot plot different shape curves by knowing only one point.

 

[vikz]

I would add looking at the owner's manual and carefully matching the requirements of the manufacturer. My Ford's c-6 trans and my minivan trans had very specific specs and special fluid needs.

I asked this not to interchange these two fluids.

No.

The relationship between viscosity and temperature is a curve. The shape of which varies depending on formulation.

You cannot plot different shape curves by knowing only one point.

Thank you for your clarification. So you mean the only way we can compare the thickness of the oil is by using the viscosity index (VI)?

One more extra question.

If the oil viscosity (cSt) at 40°C is 34 -36 range and has a higher viscosity index like 174, can such oil perform well in lower and high operating temperatures?

 

[Deleted member 89374]

@vikz plug your numbers in this calculator: https://fluidpower.pro/temperature-viscosity-chart/

This will offer you some predictability. However, when you venture into temperatures below zero Celsius, the accuracy of this calculator goes out the window. However, it's a decent tool to illustrate the curve that @Astro14 mentioned.

Also, @Astro14 mentioned that it's formulation dependant. For example an oil blended with a majority Group III base will have a vastly different curve than an oil blended with a majority PAO base.